In the throttling valve of the mentioned reference, the apparatus functions quite well. However, the improvement of the present invention enables greater life to be obtained from the valve seal mechanism. The seal mechanism is substantially improved over the apparatus shown in the referenced patent in the provision of a seal mechanism spaced between a shoulder on the valve stem proper and the valve seat, the seal mechanism including a wedge-shaped member which is driven into a softer encircling seal. The relative material hardness causes the seal ring to expand radially inwardly and outwardly, thereby creating a snug seal around the stem and preventing leakage along the stem. When the device is opened, the wedge-shaped member is relaxed, thereby enabling the seal member to relax and the radial force between the seal and the stem is reduced. This, then, enables flow through through the throttling valve along the stem. The seal, wedge member, and shoulder on the stem are arranged above a seat member formed of harder material to provide a means of closure for the apparatus.